Modifying a Video's Shape (?)

I'm not sure how to accurately phrase this question, so all my searches have failed.
I have a 720x480 video clip and want to give in a polygonal shape. Not morphing or stretching the clip, but cropping the edges at an angle to make it look like a rectangular polygon while maintaing the footage.
I'm sure this a horribly simple task, but I'm still earning the ins and outs of AE.

Track mattes are very powerful, you should definitely learn about them. We often create track mattes by using a precomped copy of the footage then we effect the heck out of the footage in the precomp to make it high contrast in just the right way.